Process Downloads really on media DCs, including for the main dc_id (fix

 #261)

src/Client.Helpers.cs

@@ -314,7 +314,7 @@ public async Task<string> DownloadFileAsync(Document document, Stream outputStre
 		public async Task<Storage_FileType> DownloadFileAsync(InputFileLocationBase fileLocation, Stream outputStream, int dc_id = 0, long fileSize = 0, ProgressCallback progress = null)
 		{
 			Storage_FileType fileType = Storage_FileType.unknown;
-			var client = dc_id == 0 ? this : await GetClientForDC(dc_id, true);
+			var client = dc_id == 0 ? this : await GetClientForDC(-dc_id, true);
 			using var writeSem = new SemaphoreSlim(1);
 			bool canSeek = outputStream.CanSeek;
 			long streamStartPos = canSeek ? outputStream.Position : 0;
@@ -347,7 +347,7 @@ async Task<int> LoadPart(long offset)
 					}
 					catch (RpcException ex) when (ex.Code == 303 && ex.Message == "FILE_MIGRATE_X")
 					{
-						client = await GetClientForDC(ex.X, true);
+						client = await GetClientForDC(-ex.X, true);
 						fileBase = await client.Upload_GetFile(fileLocation, offset, FilePartSize);
 					}
 					catch (RpcException ex) when (ex.Code == 400 && ex.Message == "OFFSET_INVALID")

src/Client.cs

@@ -240,31 +240,43 @@ public void Reset(bool resetUser = true, bool resetSessions = true)
 
 		private Session.DCSession GetOrCreateDCSession(int dcId, DcOption.Flags flags)
 		{
-			if (_session.DCSessions.TryGetValue(dcId, out var dcSession))
+			if (_session.DCSessions.TryGetValue(dcId, out var dcSession) && dcSession.AuthKey != null)
 				if (dcSession.Client != null || dcSession.DataCenter.flags == flags)
 					return dcSession; // if we have already a session with this DC and we are connected or it is a perfect match, use it
+			if (dcSession == null && _session.DCSessions.TryGetValue(-dcId, out dcSession) && dcSession.AuthKey != null)
+			{
+				if (dcSession.DataCenter.flags == flags && _session.DCSessions.Remove(-dcId))
+					return _session.DCSessions[dcId] = dcSession; // we found a misclassed DC, change its sign
+				dcSession = new Session.DCSession { Id = Helpers.RandomLong(), // clone AuthKey for a session on the matching media_only DC
+					AuthKeyID = dcSession.AuthKeyID, AuthKey = dcSession.AuthKey, UserId = dcSession.UserId };
+			}
 			// try to find the most appropriate DcOption for this DC
-			if ((dcSession?.AuthKeyID ?? 0) == 0) // we will need to negociate an AuthKey => can't use media_only DC
+			if (dcSession?.AuthKey == null) // we'll need to negociate an AuthKey => can't use media_only DC
+			{
 				flags &= ~DcOption.Flags.media_only;
-			var dcOptions = _session.DcOptions.Where(dc => dc.id == dcId).OrderBy(dc => dc.flags ^ flags);
+				dcId = Math.Abs(dcId);
+			}
+			var dcOptions = _session.DcOptions.Where(dc => dc.id == Math.Abs(dcId))
+				.OrderBy(dc => dc.flags.HasFlag(DcOption.Flags.media_only) ^ (dcId < 0)).ThenBy(dc => dc.flags ^ flags);
 			var dcOption = dcOptions.FirstOrDefault() ?? throw new WTException($"Could not find adequate dc_option for DC {dcId}");
 			dcSession ??= new Session.DCSession { Id = Helpers.RandomLong() }; // create new session only if not already existing
 			dcSession.DataCenter = dcOption;
 			return _session.DCSessions[dcId] = dcSession;
 		}
 
 		/// <summary>Obtain/create a Client for a secondary session on a specific Data Center</summary>
-		/// <param name="dcId">ID of the Data Center</param>
-		/// <param name="media_only">Session will be used only for transferring media</param>
+		/// <param name="dcId">ID of the Data Center (use negative values for media_only)</param>
 		/// <param name="connect">Connect immediately</param>
 		/// <returns>Client connected to the selected DC</returns>
-		public async Task<Client> GetClientForDC(int dcId, bool media_only = true, bool connect = true)
+		public async Task<Client> GetClientForDC(int dcId, bool connect = true)
 		{
 			if (_dcSession.DataCenter?.id == dcId) return this;
 			Session.DCSession altSession;
 			lock (_session)
 			{
-				altSession = GetOrCreateDCSession(dcId, _dcSession.DataCenter.flags | (media_only ? DcOption.Flags.media_only : 0));
+				var flags = _dcSession.DataCenter.flags;
+				if (dcId < 0) flags = (flags & DcOption.Flags.ipv6) | DcOption.Flags.media_only;
+				altSession = GetOrCreateDCSession(dcId, flags);
 				if (altSession.Client?.Disconnected ?? false) { altSession.Client.Dispose(); altSession.Client = null; }
 				altSession.Client ??= new Client(this, altSession);
 			}
@@ -276,7 +288,7 @@ public async Task<Client> GetClientForDC(int dcId, bool media_only = true, bool
 				{
 					Auth_ExportedAuthorization exported = null;
 					if (_session.UserId != 0 && IsMainDC && altSession.UserId != _session.UserId)
-						exported = await this.Auth_ExportAuthorization(dcId);
+						exported = await this.Auth_ExportAuthorization(Math.Abs(dcId));
 					await altSession.Client.ConnectAsync();
 					if (exported != null)
 					{